Usage of Special Fields in HCM P&F.

As explain in another post, you can add and read some value in backend by appending them into SPECIAL_FIELDS under method IF_HRASR00GEN_SERVICE~GET_SPECIAL_FIELDS.

You can append as much field as you like into SPECIAL_FIELDS table; rule of thumb is never append any service field. 

Reason being is that any fields that appear as SPECIAL_FIELDS, you will not be to change the fields' value in INITIALIZE or DO_OPERATION methods.

To edit your logic in generic service, use SE24 Class Builder or put in your generic service BAdI name in SE19 and use forward navigation to modify the logic.